When the weather gets cold, discovering that your furnace won’t turn on can be disconcerting. If your furnace isn’t working, it could result in frozen pipes and costly repairs to your home. The most common reasons your furnace isn’t turning on are restricted airflow, electrical malfunctions, fuel line problems, and mechanical issues.
Before you start to panic, take the time to read this guide. We’ll walk you through a few simple things you can check on your own that may resolve the issue and provide some helpful furnace troubleshooting tips. If none of these checks solve the problem, it’s time to call in a professional. You can use our handy tool to find a top local HVAC company to help repair or install a new furnace.
How Does a Furnace Work?
Before we get into the possible problems that might be preventing your furnace from working correctly, we should review the basic operating principles of a home furnace so we understand where the root cause may lie.
Natural gas furnaces are surprisingly complex, with many components that work together to provide your home with warm air. Cold air enters the furnace through an intake, passing through an air filter that removes dust and airborne debris from the air and keeps the indoor air safe to breathe. The air then passes through a heat exchanger, where it gets heated by ignited fuel before being expelled through the supply air duct by the blower motor.
The entire process is controlled by a thermostat in the living area of your house, which communicates electronically with your furnace’s control board. Before we go any further, make sure your thermostat is on. No, seriously — go check. We can’t tell you how many stories we’ve heard from HVAC techs who arrived at a furnace service appointment only to discover that the furnace wasn’t working because the thermostat switch was off.
With that out of the way, let’s look at the main culprits that may cause a malfunctioning furnace.
4 Reasons Your Furnace Won’t Turn On
The four main causes of a furnace that won’t turn on are restricted airflow, electrical malfunctions, fuel line problems, and mechanical issues. Let’s start by talking about airflow.
Airflow Issues
Your furnace’s filter and ducts are a good place to start troubleshooting. All furnaces have a safety mechanism that will trigger a shutdown if poor airflow causes the temperature in the heat exchanger to reach unsafe levels.
Clogged HVAC Filter
Locate the intake on your furnace and look for the removable filter. Most filters slide out with little effort. If your filter is clogged with dust, hair, and other airborne debris, it’s a sign that you should replace it. You can contact your HVAC provider for help ordering the right replacement filter, or you can use the serial number printed on the filter to order a new one directly from the manufacturer in some cases.
When you’re installing the new filter, make sure you have it in the right orientation. Most furnace air filters have an arrow that you have to align with the airflow direction. If your furnace works as expected after replacing the filter, it probably wasn’t turning on due to low airflow. If you still have an issue, your problem might be electrical.
Blocked Ductwork
If you still have airflow issues after replacing your air filter, consider checking the inside of your ductwork, too. Check that all registers are fully open, and look down the ducts with a flashlight to see if there is a lot of dust and debris. If it looks like it is filled with dust, hire a duct cleaning company to clear the clogs.
Electrical Issues
Your furnace might also not be turning on due to a number of electrical problems. The main electrical components that are worth checking include the following:
- Your thermostat
- The ignitor
- The flame sensor
- The control board
Thermostat Settings
First, check your thermostat – make sure it is set to “On” and is in heating mode. If your thermostat is displaying an error code or is not showing anything on the display at all, I suggest hiring an HVAC technician to diagnose and fix it.
Furnace Power Switch
If your thermostat is on, the next step to troubleshooting potential electrical problems is to ensure the furnace power switch is also on. If your thermostat and furnace switches are set to on, but your furnace isn’t turning on, the ignitor may be the problem.
Ignitor Issues
Unfortunately, it’s hard to differentiate between ignitor issues and other electrical problems when your furnace isn’t turning on. If you hear the blower come on and hear the click of the gas valve opening but don’t see any flames, it’s probably due to a faulty ignitor.
Failing Flame Sensor
Another potential source of electrical trouble is the flame sensor. Furnaces use a flame sensor to detect when it is safe to continue pumping gas through the furnace. If your furnace is not burning gas, you don’t want it to continue supplying fuel to the system, as excess gas will build up and lead to an extremely dangerous situation.
Malfunctioning Control Board
Finally, if it’s not the ignitor or the flame sensor, it might be the control board — your furnace’s brain, so to speak. Control board issues usually require a complete furnace replacement, so we recommend calling your HVAC company and scheduling a service appointment if you can’t find another root cause. Control board work is beyond most homeowners — unless you have experience working on circuitry — and requires professional attention.
Your furnace’s gas valve also has a circuit board component, but we’ll discuss that in the next section.
Fuel Issues
If you’ve eliminated airflow issues and electrical malfunctions as possible causes, it’s time to look at fuel.
Closed Gas Valve
Before checking your fuel tank or inspecting fuel lines, ensure the furnace gas valve is open. It sounds silly, but you could save yourself a ton of aggravation if it’s something as easy as a closed valve.
Malfunctioning Gas Valve Controller
Even after you’ve checked that the valve is open, that doesn’t mean you don’t have another valve issue. The gas supply valve is connected to its own circuit board that tells the valve when to begin supplying gas to the burner. If this circuit board is damaged, the blower will come on, and the ignitor will trigger, but the furnace won’t turn on because no fuel is coming in.
Another easy thing to check is the fuel level. Your fuel supplier should schedule deliveries at the appropriate frequency to avoid running out, but sometimes higher-than-normal usage or a gas leak can cause you to run out before your scheduled delivery.
Clogged Fuel Line
If your tank isn’t empty and your valve is triggering correctly, you might have a clogged fuel line. Unfortunately, there’s not much you can do yourself to combat a clogged line. Your best option is to call an HVAC professional and have them inspect the line for clogging, blockages, and leaks.
Dirty Burner
The last fuel-related furnace issue you should look for is a dirty burner. Keeping your furnace’s burner clean is important to keep it functioning efficiently. A can of compressed air and a small brush are all you need to dislodge any gunk or dust that has built up in the burner.
Make sure your furnace is powered off before removing the burner cover. Take the burner out and remove any visible deposits with your brush. Use your can of compressed air to remove any debris from hard-to-reach areas.
Mechanical Issues
The final class of problems you might encounter is mechanical issues. We already mentioned that you should ensure your thermostat is on before looking for a more complicated problem, but it could also be that your thermostat needs replacing.
Broken Thermostat
If you’ve eliminated other causes, it might be worth a try since a new thermostat is easy to replace yourself. You should also check your circuit breaker’s electrical panel to make sure that the circuit your furnace is on isn’t tripped.
Failing Burner Pressure Switch
Another mechanical component that can fail is your burner’s pressure switch. A pressure switch failure is often mistaken for a valve problem since the gas valve won’t open, and you won’t hear it click if the pressure switch is malfunctioning. If you have a voltmeter, you can check the pressure switch — with your furnace off, of course — to see if it’s shorted.
Faulty Limit Switch
On a similar note, all furnaces have a limit switch — a built-in safety feature that prevents the temperature in the burner from getting too high. If this safety switch fails, your furnace won’t turn on since operating without it is dangerous. Testing the limit switch with a voltmeter is an easy way to tell if it needs to be replaced.
Repairing a Furnace That’s Not Turning On
If you have some experience working with furnaces, you might be tempted to try and diagnose and fix the problem yourself. While this can seem like a good idea when you’re cold, ensure you understand the terms of your furnace’s service contract if you have one.
In many cases, working on your furnace yourself will void your warranty, and you’ll be on the hook for any future repairs yourself. We only recommend DIY work on your HVAC system if you’re a licensed service technician — and even then, you may still void your contract.
If you are a licensed furnace tech and want to give the DIY route a try, below are some easy fixes you can do yourself with only minimal equipment and experience:
- Clean the air filter
- Clean the burner
- Get a new thermostat
- Replace the ignitor
- Replace the pressure switch
More complicated furnace repairs like replacing the control board, clearing the gas line, and replacing the flame sensor are best left to a professional. A professional HVAC tune-up will also make sure that your furnace is in good shape overall and can help you avoid problems in the future.
When To Call a Professional
Once you’ve done your due diligence and eliminated the easy fixes, it’s time to make a service call. Problems like clogged fuel lines, faulty control boards, and leaking furnaces aren’t worth messing with yourself. Unless you have experience as a plumber or HVAC technician, chances are you’ll do more harm than good and wind up with an expensive repair visit anyway. If you need help finding the best HVAC professionals in your area, check out our locator tool.
FAQs About Your Furnace Not Working
What are some signs that my furnace is about to fail?
The telltale signs of a failing furnace are a slow start, inability to reach the desired temperature setting, unusual smells, discolored pilot light, and noisy operation. If your room temperature is lower than usual despite your thermostat setting, it’s a good sign that you need a furnace maintenance visit.
What are some common causes of furnace breakdowns?
The most common causes of furnace breakdowns are a malfunctioning thermostat; a clogged air filter or intake vent; electrical malfunctions of the control board, pressure switch, or flame sensor; and clogged or leaking fuel lines. If you have an electric furnace, faulty heating coils are one of the most common reasons for a breakdown.
Some programmable thermostats — such as those that often come with heat pumps — can also fail, giving homeowners the impression that their furnace needs work when, in reality, they just need to replace the thermostat.
What should I do if my furnace gives off an unpleasant smell?
If you notice any changes in your furnace’s sounds or smells, call a technician immediately. Failing furnaces sometimes leak carbon monoxide, so waiting and seeing if the problem worsens isn’t safe.
Why is my furnace blowing cold air?
If your furnace is blowing cold air as if it were an air conditioning system, it could be that gas is not being supplied to the burner. Without a fuel source, your furnace won’t be able to produce heat. It could also be an issue with the furnace flame sensor triggering your furnace to shut down when it shouldn’t. Airflow issues and clogged gas lines can also result in cold air but are more likely to prevent your system from turning on at all.
How often should I replace my furnace filter?
You should replace your heating system’s air filter at least twice yearly, but every three months is even better. You can also inspect your air filter occasionally and replace it when it gets clogged. A furnace that starts running more frequently is also a common sign that you might need to replace a dirty air filter.
